El cabecero de madera queda mejor junto a la pared blanca que el antiguo.

In this sentence, quedar means something like to look / to fit / to work visually in a certain place.

So El cabecero de madera queda mejor junto a la pared blanca means the wooden headboard looks better or goes better next to the white wall.

• quedar bien / quedar mejor = to look good, suit, fit well in a context
• ser mejor = to be better in a more general sense

So:

• Es mejor = it is better
• Queda mejor = it looks better / works better visually

Here the speaker is talking about appearance or design, so queda mejor is the natural choice.

Cabecero usually means headboard, especially the headboard of a bed.

In Spain, cabecero is a very common word in furniture and interior design contexts.

So el cabecero de madera = the wooden headboard.

In Spanish, the material something is made of is very often expressed with de + material.

So:

• de madera = made of wood
• de metal = made of metal
• de cristal = made of glass

That is why cabecero de madera means wooden headboard.

This is often more natural in Spanish than using an adjective equivalent to the English material adjective.

Because de madera is a standard way to describe material, and in this use Spanish normally does not use the article.

Compare:

• una mesa de madera = a wooden table
• una puerta de metal = a metal door

You would only add an article if you were referring to a specific wood in a more specific context, but here it is just describing what the headboard is made of.

Junto a means next to or beside.

Yes, al lado de would also work in many situations:

• junto a la pared blanca
• al lado de la pared blanca

Both mean roughly next to the white wall.

A small nuance:

• junto a can sound a little more compact or slightly more formal/written
• al lado de is very common in everyday speech

In this sentence, junto a sounds very natural.

In Spanish, adjectives usually come after the noun, so:

• la pared blanca = the white wall

Putting the adjective before the noun is possible in some cases, but it often sounds more literary, emphatic, or stylistically marked.

So for a normal descriptive sentence, la pared blanca is the standard order.

Also, blanca agrees with pared, which is feminine singular:

• pared = feminine singular
• blanca = feminine singular

This is a comparison, and Spanish often leaves out repeated words when they are obvious from context.

The full idea is:

• El cabecero de madera queda mejor junto a la pared blanca que el cabecero antiguo.

But repeating cabecero is unnecessary, so Spanish says:

• ...que el antiguo

Here el antiguo means the old one.

So antiguo is acting like a noun because the article el turns the adjective into the old one.

This is very common in Spanish:

• el rojo = the red one
• la grande = the big one
• los nuevos = the new ones

Here it means old.

That is because el antiguo is standing in for el cabecero antiguo = the old headboard.

A useful point: antiguo can change nuance depending on position.

• un amigo antiguo = an old friend, usually in the sense of aged or from long ago
• un antiguo amigo = a former friend

But in this sentence, el antiguo is most naturally understood as the old one.

Because in Spanish, comparatives like mejor, peor, mayor, menor, más, and menos are commonly followed by que when comparing two things.

So:

• mejor que = better than
• más grande que = bigger than
• menos caro que = less expensive than

In this sentence:

• queda mejor ... que el antiguo = looks better ... than the old one

That is the normal comparative structure.

It is comparing two headboards.

The structure is:

• The wooden headboard looks better next to the white wall than the old one does.

So the shared idea junto a la pared blanca applies to both items being compared.

In other words, the comparison is:

• el cabecero de madera vs.
• el antiguo

not:

• junto a la pared blanca vs. something else

Spanish often leaves out repeated material in comparisons, just like English does.

Yes, in many contexts you could.

• se ve mejor = looks better
• queda mejor = looks better / suits better / fits better

The difference is small, but queda mejor often suggests that something works better aesthetically in that setting. It is very common when talking about clothes, furniture, decoration, colours, and arrangement.

So here:

• El cabecero de madera se ve mejor... = understandable
• El cabecero de madera queda mejor... = especially natural for décor/design

There are a few useful agreement points:

• El cabecero → masculine singular
• de madera → no agreement here, because it is a de + noun phrase
• la pared blanca → pared is feminine singular, so blanca is feminine singular too
• el antiguo → masculine singular, because it refers back to cabecero

So although madera and pared are feminine nouns, antiguo is masculine because it refers to cabecero, not to pared.

No, not in this sentence.

You need mejor because this is a true comparison: one thing looks better than another.

• queda mejor = looks better

Más bien usually means something like rather or more accurately, and it is used differently:

• No es azul, más bien verde. = It’s not blue, rather green.

So here mejor is the correct word.